A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
For manufacturing different designs of integrated circuits different patterning devices (masks or reticles) have to be made. In the case of design errors new patterning devices have to be made. This contributes significantly to device costs, especially when small series of integrated circuits have to be manufactured. Accordingly, there have been attempts to provide for rewriteable patterning devices.
U.S. Pat. No. 6,819,469 discloses a “light modulator” for a 3D holographic display. The document mentions that the light modulator can also be used for lithography. This light modulator comprises a surface with one or more layers of phase-change material thereon, which can be brought from a crystalline to an amorphous state and back. Various methods for effecting such a phase change are described, including use of electrodes on the layer and optical heating. The phase-change layer can be used to modulate reflection using interference between light reflected from different surfaces of the layer.
In order to reduce integrated circuit feature size it is desirable to reduce the wavelength of radiation used to transfer the pattern from the patterning device to substrates. Wavelengths as far down as the EUV range have been proposed for this purpose. The light modulator of U.S. Pat. No. 6,819,469 is intended for use in the visible part of the spectrum. It is difficult to apply the described type of “light modulator” for shorter wavelengths, such as wavelengths in the EUV range. For one reason this is due to less than optimal optical properties of phase-change materials at shorter wavelengths. Also it is difficult to write patterns into the light modulator with sufficient detail.